water
Article
Water/Cement/Bentonite Ratio Selection Method for Artificial
Groundwater Barriers Made of Cutoff Walls
Cristian-S, tefan Barbu
, Andrei-Dan Sabău
, Daniel-Marcel Manoli * and Manole-Stelian S, erbulea
Department of Geotechnical and Foundation Engineering, Technical University of Civil Engineering Bucharest,
020396 Bucharest, Romania; cs.barbu@gmail.com (C.-S, .B.); andreisabau228@gmail.com (A.-D.S.);
manole.serbulea@utcb.ro (M.-S.S, .)
* Correspondence: daniel.manoli@utcb.ro; Tel.: +40-723-166-182
Abstract: As urban development requires groundwater table isolation of various historically polluted
sources, the necessity of building effective, strong, flexible, and low-permeability cutoff walls raises
the question of choosing optimum construction materials. Various authors have proposed water–
cement–bentonite mixtures, which are often chosen by experience or a trial-and-error approach,
using classical methods for testing (Marsh funnel) and representation of results (water–cement ratio,
water–bentonite ratio). The paper proposes a more precise approach for assessing the viscosity and
global representation of the three components. Moreover, this approached is exemplified with a
better documented recipe for the choice of materials based on laboratory results. The representation
of the mixtures was undertaken on a limited domain of a ternary diagram, where the components are
given in terms of mass percentage. The derived properties (viscosity, permeability, and compressive
strength) are presented on a grid corresponding to the physically possible mixtures. Based on
this representation, the most efficient recipes are chosen. Because the mixture contains only fine
aggregates, the viscosity was determined using a laboratory viscosimeter.
Keywords: cutoff walls; plastic concrete; cement-bentonite-water ratio
Citation: Barbu, C.-S, .; Sabău, A.-D.;
Manoli, D.-M.; S, erbulea, M.-S.
Water/Cement/Bentonite Ratio
Selection Method for Artificial
1. Introduction
Groundwater Barriers Made of
Since the expansion and development of urban areas, there has been an increasing
preoccupation with the quality of the groundwater table. In order to preserve the cleanliness
of groundwater, polluting sites must be isolated by containing them via the use of perimetral
impervious barriers. In addition to the main, water-retaining function, this type of structure
may also have a structural influence on the protected buildings or sites.
This technology is being broadly used in hydrotechnical structures for seepage control,
and official guidelines already exist in certain countries, without being mandatory. The
issue is still far from being solved or regulated, and the behavior of cutoff walls depends
on the design requirements. Cutoff walls made of plastic concrete have a lesser impact on
the hydro-geological urban environment because, unlike reinforced concrete trench walls,
this type of structure may be broken in-place at the end of construction work, thus resetting
the groundwater flow.
The plastic concrete differentiation is mainly described by the water–cement–bentonite
content, because the aggregates’ role is well established. The approach of this paper is similar to the characterization of binders in regular concrete, namely, the use for testing of a standardized sand according to EN 196-1 in a ratio of 1:1 with the cement–bentonite mixture.
Depending on the design requirements, there are a range of materials or mixtures that
are used for building the barriers: soil–bentonite mixtures, soil–cement–bentonite mixtures,
or plastic concrete. Various authors have proposed a wide range of variation for the amount
of water, cement, and bentonite. In this study, we attempted to cover the proposed range as
much as possible, in correlation with our own studies, which showed that a large amount
of bentonite tends to reduce the compressive strength and increases the shrinkage-swelling
Cutoff Walls. Water 2022, 14, 376.
https://doi.org/10.3390/w14030376
Academic Editor: C. Radu Gogu
Received: 10 December 2021
Accepted: 24 January 2022
Published: 26 January 2022
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Copyright: © 2022 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
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Attribution (CC BY) license (https://
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4.0/).
Water 2022, 14, 376. https://doi.org/10.3390/w14030376
https://www.mdpi.com/journal/water
Water 2022, 14, 376
mixtures, or plastic concrete. Various authors have proposed a wide range of variation for
the amount of water, cement, and bentonite. In this study, we attempted to cover the
proposed range as much as possible, in correlation with our own studies, which showed
2 of 12
that a large amount of bentonite tends to reduce the compressive strength and increases
the shrinkage-swelling capacity during soil moisture variation. Hence, in this study we
limited the amount of bentonite to about 30%.
capacity during soil moisture variation. Hence, in this study we limited the amount of
The ratios used in this paper are given in Table 1, with a number of case histories
bentonite to about 30%.
documented
in previous
papers.
The ratios
used in this
paper are given in Table 1, with a number of case histories
documented in previous papers.
Table 1. Reference values from previous contributions.
Table 1. Reference values from previous contributions.
Water to
Water to
Cement Ratio Bentonite Ratio
Water to Cement
Water to
Paper
Reference
This
paper
1.25–3.65
2.00–10.42
Ratio
Bentonite
Ratio
David Alos S. et al., 2020 [1]
3.30–10.00
This paper
1.25–3.65
2.00–10.42
FadaieAlos
M.A.,
al.,2020
2019
1.60–2.00
8.00–20.00
David
S. etetal.,
[1][2]
3.30–10.00
PishehM.A.,
Y.P.,etetal.,
al.,2019
2018
1.80
6.30–10.20
Fadaie
[2][3]
1.60–2.00
8.00–20.00
Pisheh
Y.P.,
al., 20182014
[3] [4]
1.80
6.30–10.20
U.S.
Dept.
of et
Interior,
1–2.78
6.67–13.90
U.S. Dept. of Interior, 2014 [4]
1–2.78
6.67–13.90
Hinchberger S.D., et al., 2010 [5]
1.70–2.35
12.50–18.20
Hinchberger S.D., et al., 2010 [5]
1.70–2.35
12.50–18.20
Bagheri
A.,etetal.,
al.,2008
2008
1.80–2.60
13.00
Bagheri A.,
[6][6]
1.80–2.60
13.00
Paper Reference
Bentonite to
Cement Ratio
Bentonite to
0.175–1.25
Cement
Ratio
0.1–0.24
0.175–1.25
0.00–0.40
0.1–0.24
0.14–0.29
0.00–0.40
0.14–0.29
0.1–0.22
0.1–0.22
0.20–0.30
0.20–0.30
0.14–0.23
0.14–0.23
Because the ratio between the main components—water, cement, and bentonite—
Because the ratio between the main components—water, cement, and bentonite—
needs to be represented on a scale with three dimensions, with one linear dependency
needs to be represented on a scale with three dimensions, with one linear dependency
(percentage of cement + percentage of bentonite + percentage of water = 1), to place them
(percentage of cement + percentage of bentonite + percentage of water = 1), to place
into context, the three elements were represented on a ternary diagram using their mass
them into context, the three elements were represented on a ternary diagram using their
percentages.
mass percentages.
To highlight the area of interest, the proposed ternary diagram was limited to a
To highlight the area of interest, the proposed ternary diagram was limited to a
subdomain in which the percentual ratios characterize mixtures that are feasible for all
subdomain in which the percentual ratios characterize mixtures that are feasible for all
applications. A uniform mesh of percentage combinations was chosen because it provides
applications. A uniform mesh of percentage combinations was chosen because it provides
a smooth representation of each component effect (Figure 1).
a smooth representation of each component effect (Figure 1).
Figure1.1.Mesh
Meshofofcement–bentonite–water
cement–bentonite–watercombinations.
combinations.
Figure
The ternary subdomain is used as a planimetric representation of mixtures for which
various parameters (such as viscosity and the coefficient of permeability) are represented.
To provide a proper representation, the areas outside of the represented domain were
cropped (Figure 2). The limit lines were set based on the values of the contributions
Water 2022, 14, 376
Water 2022, 14, 376
3 of 13
The ternary subdomain is used as a planimetric representation of mixtures for which
various parameters (such as viscosity and the coefficient of permeability) are represented.
3 of 12
To provide a proper representation, the areas outside of the represented domain were
cropped (Figure 2). The limit lines were set based on the values of the contributions listed
inlisted
Table
Because
the out-of-plane
values
maymay
have
exponential
variations,
in 1.
Table
1. Because
the out-of-plane
values
have
exponential
variations,where
where
required
for
the
readability
of
the
plot,
a
logarithmic
scale
was
employed.
required for the readability of the plot, a logarithmic scale was employed.
Figure2.2.Ternary
Ternarysubdomain
subdomaincropping.
cropping.
Figure
2. Preparation of Samples
2. Preparation of Samples
For the preparation of the samples, standard sand was used because it is recommended
For the
of the for
samples,
standard
sand
was used
because
it is
by norms
thatpreparation
describe the method
determination
of the
compressive
strength
of cement
recommended
by
norms
that
describe
the
method
for
determination
of
the
compressive
mortar (EN 196-1:2016—Particle size distribution is given in Table 2). No other aggregate
strength
cement mortar
size distribution
is given
in Table
2).
gradingofdistribution
was (EN
used196-1:2016—Particle
to ensure the repeatability
of tests and
to keep
the focus
No
other
aggregate
grading
distribution
was
used
to
ensure
the
repeatability
of
tests
and
on the water active components. For each sample, the ratio of sand to hydraulic binder
to(cement
keep the
focus
on the was
water
active
components.
sample,
ratiothe
of sand
to
and
bentonite)
1:1.
The usage
of sand For
waseach
employed
to the
prevent
cracking
hydraulic
binder
(cement
and
bentonite)
was
1:1.
The
usage
of
sand
was
employed
to
of the samples that may occur due to the lack of resistance. Even if the content of sand itself
prevent
the the
cracking
of the
samples
that
may occur
due to the lack
of resistance.
if
influences
behavior
of the
overall
mixture,
the relationship
between
the massEven
of sand
the
content
of of
sand
itselfofinfluences
the overall
mixture,
relationship
and
the sum
masses
bentonite the
andbehavior
cement isofbijective.
Moreover,
thethe
amount
of sand
between
the in
mass
of sand and the
sum of masses
of bentonite
cement
is bijective.
determines
a 4D-barycentric
representation
a plane
due to theand
linearity
of the
variation
Moreover,
the
amount
of
sand
determines
in
a
4D-barycentric
representation
a
(Figure 3). For tidiness of the representation, the axis of sand was disregarded.plane due
to the linearity of the variation (Figure 3). For tidiness of the representation, the axis of
sand
disregarded.
Tablewas
2. Particle
size distribution of the CEN Standard sand.
The 1:1 ratio used herein is not generally challenged in the industry, and therefore
Square
Mesh
(mm) of this 2.00
1.00
0.50
0.16
0.08
was not
among
theSize
objectives
study. 1.60
Cumulative
(%) factor 0in the 7development
±5
33 ± 5of water–cement
67 ± 5
87 ±and
5
99 ± 1
The sand Sieve
is notResidue
an active
water–
bentonite gels. This contribution aimed at showcasing the reciprocal influence of
hydration
when
theused
solids
are simultaneously
added in the
as normally
happens
The 1:1
ratio
herein
is not generally challenged
in mixture
the industry,
and therefore
was
innot
situ.
among the objectives of this study.
The sand is not an active factor in the development of water–cement and water–
bentonite gels. This contribution aimed at showcasing the reciprocal influence of hydration
when
the Mesh
solidsSize
are (mm)
simultaneously
in the mixture
happens
in 0.08
situ.
Square
2.00 added1.60
1.00 as normally
0.50
0.16
The
cement
chosen
for
sample
preparation
was
Portland
cement
with
high
Cumulative Sieve Residue (%)
0
7±5
33 ± 5
67 ± 5
87 ± 5
99 ±initial
1
strength, having the minimum standard compressive strength of 42.5 MPa (CEM IIA 42.5R).
Sodium bentonite was used as additive in the plastic concrete mix. The water introduced
in the mixtures was regular tap water.
In previous studies, the mixing sequence implied the hydration of bentonite up to 24 h
before adding the other components [5–7], although similar testing results were obtained
by dry mixing all the parts of the mix [2]. Considering the actual site conditions, dry mixing
is closer to the technological reality and more effective in terms of time and cost. Detailing
the application of the method mentioned before, in this study the bentonite and the cement
were added together with the aggregates, followed by the addition of water. The mixture
was poured in 50 × 100 mm cylindrical plastic molds matching the triaxial test sample.
Table 2. Particle size distribution of the CEN Standard sand.
Water 2022, 14, 376
4 of 12
Water 2022, 14, 376
The
laboratory testing program started 28 days after the mixtures were poured. The time4
interval was selected in order to ensure that the plastic concrete reached its strength.
of 13
Figure 3. Ternary
diagram
in a 4D-barycentric
representation
in the plane
defined
the equation
Figure
3. Ternary
diagram in a 4D-barycentric
representation
in the
plane by
defined
by the equation
Sand = +
Mass
Bentonite + Mass
Cement.
MassSand = MassMass
Mass
.
Bentonite
Cement
Water 2022, 14, 376
The cement
for sample
preparation
was Portlandusing
cement
high initial
The determination
of the chosen
permeability
coefficient
was undertaken
thewith
constant
strength,
having
the
minimum
standard
compressive
strength
of
42.5
MPa
(CEM IIA
head permeameter method, whereas the determination of the axial compressive strength
42.5R).
Sodium
bentonite
was
used
as
additive
in
the
plastic
concrete
mix.
The
water
was performed following the norm EN ISO 17892-7:2018.
introduced
in
the
mixtures
was
regular
tap
water.
The chosen range for different components proved to be sufficient because unfavorable
In previous studies, the mixing sequence implied the hydration of bentonite up to 24
effects could be noticed at the extremes. The high content of water, despite ensuring a good
h before adding the other components [5–7], although similar testing results were
workability, led to contraction cracking of the samples (Figure 4) 5and
sedimentation of
of 13the actual site conditions,
obtained by dry mixing all the parts of the mix [2]. Considering
sand; all the sedimented
were
tested and are
marked
such
in the in
Figure
dry mixing samples
is closer to
the technological
reality
and as
more
effective
terms5.of time and
cost. Detailing the application of the method mentioned before, in this study the bentonite
and the cement were added together with the aggregates, followed by the addition of
water. The mixture was poured in 50 × 100 mm cylindrical plastic molds matching the
triaxial test sample. The laboratory testing program started 28 days after the mixtures
were poured. The time interval was selected in order to ensure that the plastic concrete
reached its strength.
The determination of the permeability coefficient was undertaken using the constant
head permeameter method, whereas the determination of the axial compressive strength
was performed following the norm EN ISO 17892-7:2018.
The chosen range for different components proved to be sufficient because
unfavorable effects could be noticed at the extremes. The high content of water, despite
ensuring a good workability, led to contraction cracking of the samples (Figure 4) and
sedimentation of sand; all the sedimented samples were tested and are marked as such in
the Figure 5.
An excessive amount of bentonite led to a crumbling behavior of the material,
whereas large amounts of cement reduced the workability of fresh mixtures.
Figure 4. Contraction cracking caused by high water content.
Figure 4. Contraction cracking caused by high water content.
401.87
200
Water 2022, 14, 376
5 of 12
Figure 4. Contraction cracking caused by high water content.
401.87
200
100
20
10
Viscosity [Pa·s]
50
5
2
1
0.68
Figure 5. Viscosity variation of the fresh mixtures and
and marks
marks of
of segregated
segregated sand
sand samples.
samples.
AnResults
excessive amount of bentonite led to a crumbling behavior of the material, whereas
3. Test
largeThe
amounts
of cement
reduced using
the workability
of viscometer
fresh mixtures.
viscosity
was measured
a rotational
to measure the torque on
rods of various geometries depending on the liquid. The slope of τ-γ’ is the dynamic
3. Test Results
viscosity considering the fresh mix as being quasi-Newtonian. As expected, the variation
The viscosity
was
measured(between
using a rotational
viscometer
to measure
torque on
in viscosity
is quite
important
0.68–401.87
Pa·s); thus,
in order the
to provide
a
rods of various geometries depending on the liquid. The slope of τ-γ’ is the dynamic
balanced representation of colors, a [0, 1] normalized logarithmic scale was employed as
viscosity considering the fresh mix as being quasi-Newtonian. As expected, the variation in
described in Equation (1) [8].
viscosity is quite important (between 0.68–401.87 Pa·s); thus, in order to provide a balanced
representation of colors, a [0, 1] normalized logarithmic scale was employed as described
in Equation (1) [8].
ln(ηi )
ln(ηi )
−
min
n
n
ln(∏i=1 ηi )
ln(∏i=1 ηi )
norm(µi ) =
(1)
ln(ηi )
ln(η )
max ln(∏n η ) − min ln(∏n i η )
i =1 i
i =1 i
For graph plotting, Python 3.9.9 and numpy, matplotlib, mpltern, and ternary libraries
were used.
As expected, the larger the ratio of water–solids, the smaller the viscosity. It should
be noted that the Marsh funnel method refers to reference values for bentonite slurry of
32 to 60 s [8], corresponding to viscosities too low to be considered for workable plastic
concrete, whereas the concrete viscosity is reported to be between 2–27 Pa·s [9,10], so that
the measured values presented herein cover the latter reported range.
The variation is progressive and smooth, so Figure 5 may be used for assessing
expected workability when preparing cement–bentonite–water mixtures with sand. It may
be noted that limiting the lower viscosity to 5 Pa·s prevents the segregation. The advantage
of the proposed plot consists also in setting the mixture without limiting the water content,
and in choosing a proper ratio among components, and an increase in bentonite solves the
sedimentation issue.
We consider the workability acceptable up to 100 Pa·s, but ideal under 50 Pa·s.
For higher values, additives such as superplasticizers are required to attain proper flow
for casting.
For choosing the best suited mixture, it is important to correlate workability with the
targeted failure behaviors and permeabilities.
Water 2022, 14, 376
water content, and in choosing a proper ratio among components, and an increase in
bentonite solves the sedimentation issue.
We consider the workability acceptable up to 100 Pa·s, but ideal under 50 Pa·s. For
higher values, additives such as superplasticizers are required to attain proper flow for
casting.
6 of 12
For choosing the best suited mixture, it is important to correlate workability with the
targeted failure behaviors and permeabilities.
The quantity of bentonite negatively impacts the compressive strength when
quantity
of bentonite
impacts
the compressive
strength
when portions
portions The
larger
than 17%
are used.negatively
In this case,
the strength
is decreased
by factors
of 2
larger
than
17%
are
used.
In
this
case,
the
strength
is
decreased
by
factors
2 and more
and more with respect to lower bentonite amounts (Figure 6). This highlights a of
boundary
with respect
to lower
bentonite
amountscontent
(Figurearound
6). This17%,
highlights
a boundary
of material
of material
behavior
at values
of bentonite
from which
the hardened
behavior
at
values
of
bentonite
content
around
17%,
from
which
the
hardened
mixtures shift from having predominant compressive strength to shearing strength;mixtures
that
shift fromfrom
having
predominant
strength
is, switching
weak
concrete tocompressive
hard soil (Figure
7). to shearing strength; that is, switching
from weak concrete to hard soil (Figure 7).
627
600
500
400
300
200
100
57.0
Water 2022, 14, 376
7 of 13
Figure 6. Axial compressive strength.
Figure 6. Axial compressive strength.
(a)
(b)
Figure
concrete
samples.
(a) (a)
“weak
concrete”—compressive
failure.
Figure7.7.Failure
Failurepatterns
patternsofofplastic
plastic
concrete
samples.
“weak
concrete”—compressive
failure.
(b)
(b) “strong
“strong soil”—shearing
soil”—shearing failure.
failure.
Conversely,
Conversely, the
the material
material permeability
permeability isis also
also decreased
decreased by
by aa factor
factor of
of 55 or
or more
more
(Figure 8)
8) along
along the
the same
same boundary
boundary of
of about
about 17%,
17%, confirming
confirming the
the model
model of
of switching
switching
(Figure
between the
the mechanical
mechanical behavior
behavior of
of the
the hardened
hardened mixture.
mixture.
between
39
35
30
25
Figure 7. Failure patterns of plastic concrete samples. (a) “weak concrete”—compressive failure.
(b) “strong soil”—shearing failure.
Conversely, the material permeability is also decreased by a factor of 5 or more
(Figure 8) along the same boundary of about 17%, confirming the model of switching7 of 12
between the mechanical behavior of the hardened mixture.
Water 2022, 14, 376
39
35
30
25
20
15
10
5
3.9
Figure 8. Permeability.
Figure 8. Permeability.
All data resulting from laboratory tests are given in Table 3. The mixtures are given
All data
resulting
from laboratory
are given
3. The
mixtures are given
both
in terms
of percentage
of each tests
component
andinasTable
classical
ratios.
both in terms of percentage of each component and as classical ratios.
Table 3. Test data table.
No.
Water
[%]
Bent.
[%]
Cem.
[%]
w/c
w/b
c/b
K
[cm/s]
cu
[kPa]
Segregated
Γ
[kN/m3 ]
µ
[Pa·s]
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
50.0
52.5
55.0
50.0
52.5
55.0
57.5
50.0
52.5
55.0
57.5
60.0
50.0
52.5
55.0
57.5
60.0
62.5
50.0
52.5
55.0
57.5
60.0
62.5
65.0
50.0
52.5
25.0
25.0
25.0
22.5
22.5
22.5
22.5
20.0
20.0
20.0
20.0
20.0
17.5
17.5
17.5
17.5
17.5
17.5
15.0
15.0
15.0
15.0
15.0
15.0
15.0
12.5
12.5
25.0
22.5
20.0
27.5
25.0
22.5
20.0
30.0
27.5
25.0
22.5
20.0
32.5
30.0
27.5
25.0
22.5
20.0
35.0
32.5
30.0
27.5
25.0
22.5
20.0
37.5
35.0
2.00
2.33
2.75
1.82
2.10
2.44
2.88
1.67
1.91
2.20
2.56
3.00
1.54
1.75
2.00
2.30
2.67
3.13
1.43
1.62
1.83
2.09
2.40
2.78
3.25
1.33
1.50
2.00
2.10
2.20
2.22
2.33
2.44
2.56
2.50
2.63
2.75
2.88
3.00
2.86
3.00
3.14
3.29
3.43
3.57
3.33
3.50
3.67
3.83
4.00
4.17
4.33
4.00
4.20
1.00
0.90
0.80
1.22
1.11
1.00
0.89
1.50
1.38
1.25
1.13
1.00
1.86
1.71
1.57
1.43
1.29
1.14
2.33
2.17
2.00
1.83
1.67
1.50
1.33
3.00
2.80
3.92 × 10−5
3.90 × 10−5
3.74 × 10−5
3.69 × 10−5
1.49 × 10−5
1.70 × 10−5
2.16 × 10−5
3.82 × 10−5
3.89 × 10−5
3.62 × 10−5
1.47 × 10−5
1.53 × 10−5
1.40 × 10−5
1.32 × 10−5
3.25 × 10−5
1.40 × 10−5
1.11 × 10−5
1.44 × 10−5
1.12 × 10−5
3.89 × 10−6
9.82 × 10−6
6.91 × 10−6
1.35 × 10−5
2.71 × 10−5
1.35 × 10−5
9.45 × 10−6
5.25 × 10−6
162
86
82
166
156
93
79
193
145
124
93
71
219
158
124
89
72
67
194
424
388
368
228
201
120
627
524
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
13.04
11.95
10.98
12.32
12.02
11.11
10.49
13.12
12.58
12.06
11.31
11.19
13.45
12.89
12.09
11.95
11.38
10.81
13.86
15.06
14.62
13.77
12.76
11.96
12.16
13.85
14.69
237.73
154.33
103.48
204.73
74.04
98.57
36.13
401.87
132.92
96.34
49.51
44.60
156.56
34.34
80.29
31.22
10.17
3.97
48.62
26.76
15.08
7.05
5.71
6.87
3.64
10.70
10.35
Water 2022, 14, 376
8 of 12
Table 3. Cont.
No.
Water
[%]
28
55.0
29
57.5
30
60.0
31
62.5
32
65.0
33
67.5
34
50.0
35
52.5
36
55.0
37
57.5
38
60.0
39
62.5
40
65.0
41
67.5
42
70.0
43
53.0
44
55.0
45
57.5
46
60.0
47
62.5
48
65.0
49
67.5
50 14, 37670.0
Water 2022,
51
72.5
52
73.0
Bent.
[%]
Cem.
[%]
w/c
w/b
c/b
K
[cm/s]
cu
[kPa]
Segregated
Γ
[kN/m3 ]
12.5
12.5
12.5
12.5
12.5
12.5
10.0
10.0
10.0
10.0
10.0
10.0
10.0
10.0
10.0
7.0
7.0
7.0
7.0
7.0
7.0
7.0
7.0
7.0
7.0
32.5
30.0
27.5
25.0
22.5
20.0
40.0
37.5
35.0
32.5
30.0
27.5
25.0
22.5
20.0
40.0
38.0
35.5
33.0
30.5
28.0
25.5
23.0
20.5
20.0
1.69
1.92
2.18
2.50
2.89
3.38
1.25
1.40
1.57
1.77
2.00
2.27
2.60
3.00
3.50
1.33
1.45
1.62
1.82
2.05
2.32
2.65
3.04
3.54
3.65
4.40
4.60
4.80
5.00
5.20
5.40
5.00
5.25
5.50
5.75
6.00
6.25
6.50
6.75
7.00
7.57
7.86
8.21
8.57
8.93
9.29
9.64
10.00
10.36
10.43
2.60
2.40
2.20
2.00
1.80
1.60
4.00
3.75
3.50
3.25
3.00
2.75
2.50
2.25
2.00
5.71
5.43
5.07
4.71
4.36
4.00
3.64
3.29
2.93
2.86
9.75 × 10−6
8.46 × 10−6
2.29 × 10−5
1.13 × 10−5
1.54 × 10−5
3.35 × 10−5
9.98 × 10−6
1.08 × 10−5
7.32 × 10−6
1.44 × 10−5
9.44 × 10−6
2.19 × 10−5
8.71 × 10−6
6.74 × 10−6
1.90 × 10−5
7.29 × 10−6
8.71 × 10−6
1.08 × 10−5
4.70 × 10−6
7.15 × 10−6
7.03 × 10−6
6.96 × 10−6
2.84 × 10−5
1.40 × 10−5
1.57 × 10−5
251
264
223
136
161
120
529
389
390
173
471
360
415
402
334
443
368
319
349
353
393
449
57
161
94
No
No
No
No
No
No
No
No
No
No
Yes
Yes
Yes
Yes
Yes
No
No
No
Yes
Yes
Yes
Yes
No
No
Yes
13.71
13.4
12.48
11.01
10.93
10.16
13.6
13.84
12.69
12.18
13.11
12.19
12.76
13.37
13.1
14.24
13.5
12.95
14.69
13.75
13.82
14.01
10.37
10.82
10.96
µ
[Pa·s]
7.31
3.81
3.79
2.03
1.65
2.94
10.79
7.14
7.58
7.40
5.44
1.90
2.10
1.43
0.68
4.68
2.83
3.93
1.65
1.34
2.10
3.48
9 of1.10
13
5.93
2.71
4. Discussion
4. Discussion
This research focused mainly on covering all the possible mixture contents as
This research focused mainly on covering all the possible mixture contents as described
described by various contributions. This led to a wide distribution of results in terms of
by various contributions. This led to a wide distribution of results in terms of the diverse
the diverse mechanical behavior. In order to aid the visual identification of the mixtures,
mechanical behavior. In order to aid the visual identification of the mixtures, a colormap
a colormap based on red–yellow–blue coding was derived, as shown in Figure 9.
based on red–yellow–blue coding was derived, as shown in Figure 9.
Figure
9. Component
distribution
colormap.
Figure
9. Component
distribution
colormap.
The three properties studied for each mixture were viscosity, strength, and
permeability. A distinctive clustering of results induced by the feature tradeoff governed
by the components may be noted in Figure 10.
Water 2022, 14, 376
9 of 12
Figure 9. Component distribution colormap.
The three properties studied for each mixture were viscosity, strength, and permegoverned by the
The three properties studied for each mixture were viscosity, strength, and
ability. A distinctive
results
induced
the tradeoff
featuregoverned
tradeoff
permeability.
A distinctiveclustering
clustering ofof
results
induced
by theby
feature
by
the components
may
noted in
10.10.
components
may
bebenoted
inFigure
Figure
Water 2022, 14, 376
10 of 13
Figure10.10.
Viscosity
vs. strength.
Figure
Viscosity
vs. strength.
The increase in bentonite amount (yellower dots in the graphs) leads to the
The increase
bentonite
(yellower
dots in
the graphs)
to the foreseeable
foreseeable
loss ofinstrength
of amount
about 1–5
times with
respect
to the leads
cement-governed
loss
of
strength
of
about
1–5
times
with
respect
to
the
cement-governed
mixtures;
however
mixtures; however the viscosity increases by about 1–1.5 orders of magnitude. The large
the viscosity
increases
aboutconcrete
1–1.5 orders
of magnitude.
large variation
strength
variation
in strength
forby
plastic
is documented
in theThe
literature,
and the in
range
for
plastic
concrete
is
documented
in
the
literature,
and
the
range
found
herein
was
found herein was confirmed by contributions such as [5,11,12]. Other authors, such as [6], confirmed by
contributions
suchresults
as [5,11,12].
such asbeing
[6], obtained
obtained
slightly
overlapping
but withOther
most authors,
of the strengths
an order slightly
of
overlapping
results
but
with
most
of
the
strengths
being
an
order
of
magnitude
higher.
magnitude higher.
Studyingthe
thevariation
variationininthe
thestrength
strengthwith
withrespect
respecttotothe
thecommonly
commonlyused
usedratios
ratios emStudying
ployed
in
engineering
practice,
namely
water/cement
and
water/bentonite,
the
employed in engineering practice, namely water/cement and water/bentonite, influence
the
of the third
component
in the mixture
may bemay
noted.
influence
of the
third component
in the mixture
be noted.
Theeffect
effectofofstrength
strength
reduction
to the
bentonite
be readily
in Figure
The
reduction
duedue
to the
bentonite
maymay
be readily
notednoted
in Figure
11, 11,
inthe
themixtures
mixturescontaining
containing
over
17.5%
active
(yellow
shaded),
in Figure
12 with
a
in
over
17.5%
active
clayclay
(yellow
shaded),
or inor
Figure
12 with
a
water/bentonite
ratio
lower
than
4. Figure
In Figure
plastic
concrete
content
water/bentonite
ratio
lower
than
4. In
11, 11,
the the
plastic
concrete
withwith
high high
content
of of
sodiumbentonite
bentoniteisisless
less
prone
develop
hydrated
clinker
bridges,
sostrength
the strength
sodium
prone
to to
develop
hydrated
clinker
bridges,
so the
dropsdrops
under
of of
samples.
These
results
are also
noticeable
in thein the
under200
200kPa
kPafor
forthis
thiscomposition
composition
samples.
These
results
are also
noticeable
grouping
markers
in in
Figure
12.12.
groupingofofyellow-shaded
yellow-shaded
markers
Figure
Figure11.
11.Strength
Strength
water/cement
ratio.
Figure
vs.vs.
water/cement
ratio.
Water 2022, 14, 376
10 of 12
Figure 11. Strength vs. water/cement ratio.
Figure12.
12.Strength
Strengthvs.
vs.water/bentonite
water/bentonite
ratio.
Figure
ratio.
Water 2022, 14, 376
We consider viscosity to be a very important feature of the fresh plastic concrete,
because this property prevents the contamination of the cast material with water or soil
from the trench walls when replacing the slurry used for excavation. The values obtained
in our tests did not exceed the limit of proper workability.
In addition to the role of thickener for the fresh mixture, bentonite prevents the proper
11 of 13
development of cement gel and is twice as expensive as the binder; however, the decrease
in permeability of the wall for low amounts of bentonite is canceled or even worsened if
thisWe
component
is used to
in be
excess.
effectfeature
is alsoof
observed
the case
of strength.
consider viscosity
a veryThis
important
the fresh in
plastic
concrete,
In
Figure
13,
the
low
scattering
of
test
data
in
the
graph
indicates
the fact that the
because this property prevents the contamination of the cast material with water or soil
from
the trench
walls
when
replacing
the slurry
used for excavation.
The values
obtained
viscosity
of the
mix
is almost
entirely
governed
by the bentonite
and
water content, with
in
our to
tests
not exceed
the limit
of proper(Figure
workability.
little
nodid
influence
from
the cement
14).
InDue
addition
to
the
role
of
thickener
for
fresh
mixture,
prevents
the
to the large number of samplesthe
and
long
testingbentonite
time required
(employing
triaxial
proper development of cement gel and is twice as expensive as the binder; however, the
test equipment), and the hardening time limitation on batching, only one sample was
decrease in permeability of the wall for low amounts of bentonite is canceled or even
tested forif each
combination,
resulting
inThis
some
lackisof
accuracy,
worsened
this component
is used
in excess.
effect
also
observedespecially
in the case for
of permeability
tests.
However,
a
trend
in
the
data
emerged
that
helps
to
narrow
the
mixture
variation
strength.
range
to a domain
to the targeted
purposes.
A future
stage
will be to
In Figure
13, the closer
low scattering
of test data
in the graph
indicates
the in
factthis
thatresearch
the
viscosity
the mix
is almost
entirely
governed
by the bentonite
andto
water
content,
with the data on
employof
three
to five
samples
of each
combination
in order
be able
to process
little
to no influence
the a
cement
(Figure
14).
a statistical
basis,from
yet for
narrower
subdomain,
as proposed in Figure 15.
Figure
Viscosity
vs. water/bentonite
ratio. ratio.
Figure13.13.
Viscosity
vs. water/bentonite
Water 2022, 14, 376
11 of 12
Figure 13. Viscosity vs. water/bentonite ratio.
Water 2022, 14, 376
12 of 13
Due to the large number of samples and long testing time required (employing
triaxial test equipment), and the hardening time limitation on batching, only one sample
was tested for each combination, resulting in some lack of accuracy, especially for
permeability tests. However, a trend in the data emerged that helps to narrow the mixture
variation range to a domain closer to the targeted purposes. A future stage in this research
will be to employ three to five samples of each combination in order to be able to process
the data on a statistical basis, yet for a narrower subdomain, as proposed in Figure 15.
Figure14.
14.Viscosity
Viscosity
water/cement
Figure
vs.vs.
water/cement
ratio. ratio.
Not recommended
due to the alteration
of mechanical properties
induced by the high content
of bentonite.
Recommended
C
Not recommended due to aggregate
seggregation
(amount of water is too high)
Figure 15. Boundaries of recommended water–bentonite–cement mixes based on the test results.
Figure 15. Boundaries of recommended water–bentonite–cement mixes based on the test results.
5. Conclusions
5. Conclusions
The plastic concrete used in urban hydrogeological barriers requires a set of properties
The
concrete
in urban hydrogeological
barriers
a the
set tradeoff
of
that plastic
sometimes
yieldsused
to contradictory
requirements. The
best requires
example is
properties
thatpermeability
sometimes yields
to contradictory
requirements.
The best
is the
between
and strength
that involves
using bentonite
as aexample
control component.
tradeoff
strength
that
bentonite
as a control
As between
shown inpermeability
this study, aand
large
amount
of involves
bentoniteusing
decreases
permeability
and thus
component.
As of
shown
in thiswall
study,
amount
of also
bentonite
decreases
permeability
efficiency
the cutoff
but,aatlarge
the same
time,
reduces
compressive
strength with
and thus
efficiency for
of the
wall
but, atofthe
time, The
also best
reduces
compressive
consequences
the cutoff
cracking
control
thesame
structure.
approach
should be to
strength with consequences for the cracking control of the structure. The best approach
should be to choose bentonite content within limits that provide satisfactory behavior in
terms of both parameters. The recommended dosage is less than 17%, as shown herein.
Even if various authors proposed wide ranges of ratios of the mixture components,
extreme values definitely lead to unfavorable effects, such as sand segregation, low
workability, or long setting time. Because sodium bentonite and cements are well
Water 2022, 14, 376
12 of 12
choose bentonite content within limits that provide satisfactory behavior in terms of both
parameters. The recommended dosage is less than 17%, as shown herein.
Even if various authors proposed wide ranges of ratios of the mixture components,
extreme values definitely lead to unfavorable effects, such as sand segregation, low workability, or long setting time. Because sodium bentonite and cements are well standardized,
we consider it possible to predefine mixture recipes that are optimal for certain purposes.
This paper does not aim to set pre-determined mixture ratios but rather to guide
the choice of plastic concrete recipe towards the targeted design purpose. As is readily
noticeable, slight variations in components may induce an important improvement or
decay in certain mechanical characteristics.
Author Contributions: Investigation, C.-S, .B.; Data curation, A.-D.S., Methodology, D.-M.M.; Software, A.-D.S.; Supervision, M.-S.S, . All authors have read and agreed to the published version of
the manuscript.
Funding: This research received no external funding.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Data is contained within the article in Table 3.
Conflicts of Interest: The authors declare no conflict of interest.
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